1. Field of the Invention
The present invention relates to the transmission of information over a digital subscriber line (DSL).
2. Description of the Related Art
The use of digital subscriber line (DSL) technology has increased in recent years, allowing high-speed (i.e., broadband) access from a central location, typically a telephone central office, to multiple subscribers in homes or offices. The particular advantage of DSL over various other forms of broadband communication is that DSL allows the use of existing copper telephone lines from the central office to the end-user, often referred to as the “local loop”, or simply “loop”. A typical DSL application is for connecting the subscriber to the Internet via the telephone central office, but other uses are possible.
Twisted pair wires have a wide frequency over which information may be transmitted. The “voice band” or “analog band” portion of the frequency spectrum ranges from between 0 and 4 kHz and, as noted above, is typically between about 3-4 kHz. This portion of the voice channel is also used for low-speed modems of the V. series (e.g., V.32, V.34, V.90, and V.92); thus, V. series modems cannot be used simultaneously with voice transmissions over the same telephone line, i.e., if the line is being used by the modem, it can't be used for telephone voice communications. DSL, on the other hand, uses higher frequency bands, for example, from 25 kHz and up to about 1.1 mHz, over the same twisted pair wires. Since this voice band and the higher DSL band are discrete frequencies, a user is able to transmit/receive data over the DSL band while simultaneously using the voice band for voice communications or V. series analog modem data communications, including facsimile transmissions.
The traditional twisted-pair copper loop has characteristics such that the signal attenuation increases along with the signal frequency. Regular phone service uses the frequency band (0 Hz to 4 kHz), which is extremely stable and reliable and is relatively insensitive to weather-related conditions. By contrast, most of the DSL services deployed in the field use the ITU G.992.1 standard with an upstream band (28.875 kHz to 138 kHz) and a downstream band (138 kHz to 1104 kHz). Weather conditions such as temperature and humidity can dramatically change the characteristics of the wires on these DSL bands between the DSL modem located at a customer premises and a DSL access multiplexer (DSLAM) at the central office. Due to the characteristics of the signal attenuation, the upstream connection may be less problematic than the downstream. But if the downstream link is broken, the DSL connection fails. Sometimes the change in characteristics may have little effect on the voice band and minor effect on the lower end of the DSL upstream frequency band, and thus, the customer's voice band channel will seem to be experiencing no difficulty while the DSL frequencies do. These characteristic changes can be of such severity that the DSL modem data connection can fail completely.
Since such loss of data communications can be catastrophic, particularly in a business situation where data transmission is critical to the business, the typical back-up alternative used when a DSL data connection fails is to connect over the voice band using a V. series modem. In addition to being a slower connection, this solution disrupts the data flow (while the V. modem connection is established) and also prohibits the use of the telephone line over which the data is being transmitted for any purpose other than the data transmission, e.g., for voice communications. In addition, this requires the user to incur the additional expense of subscribing to a dial-up service as a back-up mechanism for data transmission. Typically this back-up dial-up service will be rarely used, since optimally the DSL connection will operate properly most of the time. Thus, the user ends up paying additional money for a back-up dial-up line that may never be used or be rarely used. Accordingly, it would be desirable to have a back-up system available that would, essentially seamlessly, allow use of the voice band for voice communications while allowing simultaneous data transmission at the DSL frequencies over the same telephone line.